Author Topic: Weight and Balance  (Read 11974 times)

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Offline 631

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Re: Weight and Balance
« Reply #30 on: June 21, 2014, 10:28:55 PM »
good points on the complexity of 3D shapes and flat side surfaces.  The on course airflow may not agree precisely with the paper area calculations for a particular vehicle and all of our cars are going to have some divergence from our simple calculated results.  The point of the exercise is to get a base line idea of the high speed stability of our cars.  Wind tunnel time will validate (or not) our stability assumptions but not all of us have access to these great devices.  Grease dots are a time honored land speed method of seeing where the air goes around a vehicle at speed, taped yarns and an on board camera are the information age replacement for grease dots,  suspension data acquisition is great info as well; stiff wire, zip ties and tape can provide simple data in place of sensors.  Weigh the car, figure out the area center of the side of the vehicle, do the cut out test then make some assumptions and next summer spend a few work up or test runs proving or disproving the stability assumptions. The 411 streamliner was designed by a young mechanical engineer using paper and pencil proving to me that good results are possible applying the basic principles of mass in motion traveling through the air.   

Offline killajoule

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Typo in calculation/equation. (Re: Weight and Balance)
« Reply #31 on: June 21, 2014, 11:51:37 PM »
Eva Hakansson here.

Sumner - you are absolutely correct. There is a typo in the calculation that carried through and the equation is flipped (it is correct in the text and wrong in the equation). Sorry about that.
It's the result of late night typing. For some reason, making a living always gets in the way of racing and you end up working late nights...

The example calculation was a fictitious example we made up for the article. I have all the calculations for the KillaJoule, but we decided to not use that as an example because the three wheels and the asymmetric body makes it a bit more complicated.

I have sent a revised version to Rex Svoboda. I hope it the current version will be replaced soon.

// Eva


Note the article on CP CG in the tech & FAQ section.

Thanks, that is a good article ....

http://www.landracing.com/docs/CG-CP_Hakansson-Dube_4-2014.pdf

and is a little more technically orientated finding the CG vs. placing two jacks under the car on each side to find where it balances and where the CG should be above that point.  If I had weight scales available I'd try that and compare it to the 'jack method'.

=======================================================

SSS if you can you might want to fix a small mistake though. There was a mistake in the part on figuring the CG.  It should be....

Rear Left     117.25 ...... 1130 ....... 132492.5  (not 112492.5)

Total ......................... 4674 ....... 292446.0 

292446.0 [inch-lbs.]/4674 [lbs.] = 62.56 [inches]  

I also turned the numerator and denominator around in the line above as that is the way I'm use to seeing them, but maybe I'm wrong,

Sum
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Offline Sumner

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Re: Typo in calculation/equation. (Re: Weight and Balance)
« Reply #32 on: June 22, 2014, 12:00:13 AM »
Eva Hakansson here. ...The example calculation was a fictitious example we made up for the article. I have all the calculations for the KillaJoule, but we decided to not use that as an example because the three wheels and the asymmetric body makes it a bit more complicated.....

Thanks for taking the time to do the article.  I think it explained everything very well and hope others take the time to read it,

Sum

Offline killajoule

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CP-CG - simple is better than nothing... (Re: Weight and Balance)
« Reply #33 on: June 22, 2014, 12:15:10 AM »
Thanks, Sum.
The article is just a "rule of thumb" type guide, but we find the method very useful.
Most of us Bonneville racers will never get anywhere near a windtunnel. If this simple method tells you that your CG is behind your CP, then you will know why your vehicle was handling badly. There is no point spending money on windtunnel time until you have made some attempt to fix the CG-CP issue.

// Eva

Eva Hakansson here. ...The example calculation was a fictitious example we made up for the article. I have all the calculations for the KillaJoule, but we decided to not use that as an example because the three wheels and the asymmetric body makes it a bit more complicated.....

Thanks for taking the time to do the article.  I think it explained everything very well and hope others take the time to read it,

Sum
Gasoline?! That is _so_ last century!
KillaJoule - FIM/AMA/LTA record holder, the world's fastest electric motorcycle and the world's fastest sidecar motorcycle. Official record 240.726 mph (AMA, Aug 2014, pending ratification), recorded top speed so far 270.224 mph, but we are just getting started!

www.killacycleracing.com
www.facebook.com/killacycle
www.youtube.com/killacycle

Offline SPARKY

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Re: Weight and Balance
« Reply #34 on: June 22, 2014, 08:29:56 AM »
"There is no point spending money on windtunnel time until you have made some attempt to fix the CG-CP issue. "


Maybe that should depend on what speeds one is trying to run---a lot of the faster cars have had frt. end lift issues---some very successful ones ---have had body changes in route to that success

some have just wrecked without changes---if the ride just gets better and better---it may not be a smoother track  :-o
« Last Edit: June 23, 2014, 08:50:25 AM by SPARKY »
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Offline wobblywalrus

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Re: Weight and Balance
« Reply #35 on: June 23, 2014, 12:38:43 AM »
One problem with having a lot of weight back past the rear axle or near it is how the vehicle reacts to bumps or dips on the track.  Weight behind the axle can actually lift the front end of the vehicle if it hits a bump.  My experience with this is with pick-up trucks overloaded with fire wood.  It is not racing, although the principles are the same.   

Offline RidgeRunner

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Re: Weight and Balance
« Reply #36 on: June 23, 2014, 07:27:29 AM »
     A buddy who had a general repair garage put together his own roll back truck back in the early '70s using a truck with a marginal WB.  Having borrowing it for a move once, I can testify that the results were the same on a larger scale, front end could get scary light real fast.  He didn't use it any more that he had to until he could get it on a truck with a longer WB.

              Ed

Offline SPARKY

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Re: Weight and Balance
« Reply #37 on: June 23, 2014, 09:10:20 AM »
On my car I start out with most of my cooling water 20" behind the rear axle---during the run I actually move about  80# from behind the rear axle to 108" in front which really helps on trying to get CG more in front of the CP.
Miss LIBERTY,  changing T.K.I.  to noise, dust and RUST!!!

The # 1 issue is: TO KEEP THE REPUBLIC      
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"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

Offline maj

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Re: Weight and Balance
« Reply #38 on: June 23, 2014, 08:08:20 PM »
My only experiance with this sort of thing is 10 yrs in dirttrack sedans, and how relevant it is to salt is anyones guess
But it left an permanant impression of weight in front of rear axle = good
and weight behind axle =bad
So much easier to steer in a slide if the back is not acting like a pendulum, leverage over contact point sort of feel

I still apply this to the bike rear fairing i had previously built and in the process of doing again, i feel it needs to be long for better aero shape minimising separation , so i put the tire right back out there too and this gives me more room to put weight in front of the tire, and transfers more of the total to the front tire   

Offline Richard 2

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Re: Weight and Balance
« Reply #39 on: June 23, 2014, 09:41:06 PM »
I found this 101 site on CP and CG of rocket stability and thought I would share it.
Shows a simple way to find CP and why it is necessary to find it in relationship to CG.
219.648 mph F/BFMR 2010 Record
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Could of had a V8

Offline Graham in Aus

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Re: Weight and Balance
« Reply #40 on: June 23, 2014, 11:38:28 PM »
« Last Edit: June 23, 2014, 11:43:00 PM by Graham in Aus »

Offline wobblywalrus

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Re: Weight and Balance
« Reply #41 on: June 23, 2014, 11:53:33 PM »
Sparky, that is smart thinking.

Greg, I read somewhere that when one increases the wheelbase the front wheel trail needs to be increased, too.  The trail vs wheelbase on the Triumph was 8% before and after it was lengthened.  It was a lot of work to increase the trail and it was harder than stretching the wheelbase.  Was what I did really needed?   


 

Offline jbryant200

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Re: Weight and Balance
« Reply #42 on: June 24, 2014, 11:54:46 AM »
How do you calculate the center of pressure?

The center of pressure is complex. There are considerations that include strakes, whether or not you have open wheels (if you do, BOTH sides of the wheels are included in the calculation), wings, spoilers, and other items not included in the rough estimate using a profile.  The profile approach, as already discussed in this forum, is a surprisingly good estimate.  There are a number of us in the LSR community that are willing to provide an unofficial calculation for those interested. We are also willing to provide additional feedback depending on your handling experiences, speed goals, or plans for body changes.

I have found that the LSR community is truly a supportive network. We want to be a part of this, especially since we have seen such a large increase in worldwide interest. There are many new projects in the pipeline and we hope that forums like this serve to help these new teams learn the safest ways to approach design.  Help get the word out!

Offline SPARKY

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Re: Weight and Balance
« Reply #43 on: June 24, 2014, 12:13:41 PM »
"I have found that the LSR community is truly a supportive network."

This is so true  you will find there are LOTS of folks that will keep you from making "numbing" rookie mistakes!!  LSR  is
" SO non COOKIE CUTTER"  as some one has as on their sig.

"If the objective is to skin a cat---SO LITTLE TIME SO MANY CATS"

We are all dealing with exactly the same thing:

  " TOTAL DRAG VS TRACTIVE EFFORT"  :cheers:
Miss LIBERTY,  changing T.K.I.  to noise, dust and RUST!!!

The # 1 issue is: TO KEEP THE REPUBLIC      
           tncsg.org     mrspowell.org

ELECTION  INTEGRITY  PROJECT
eip-ca.com  EIPAz.org  eipnv.

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

Offline Richard 2

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Re: Weight and Balance
« Reply #44 on: June 24, 2014, 06:04:10 PM »
I found this 101 site on CP and CG of rocket stability and thought I would share it.
Shows a simple way to find CP and why it is necessary to find it in relationship to CG.

http://www.rockets4schools.org/images/Basic.Rocket.Stability.pdf

Forgot the address, up to late I guess.

Richard 2
219.648 mph F/BFMR 2010 Record
4 cylinder Esslinger
Could of had a V8